498 



CEMENTS, LIMES, AND PLASTERS. 



the stack-gases would be cold, and the clinker heat would be utilized. 



In actual practice, however, a very large amount of heat is carried 

 out with the stack-gases, radiated from the exposed surfaces of the 

 kiln, and carried out in the hot clinker. 



Heat utilized in evaporation of water. It is obvious that any water 

 contained in the charge must be evaporated, and the steam thus formed 

 must be raised to the temperature of the stack-gases. It is here that 

 the great difference in economy between the dry and wet methods 

 of mixing is shown. 



In the dry method the total water (mechanically held and combined) 

 contained in the charge will rarely rise above 2^ per cent, of which 

 about 2 per cent may be combined in the clay and ^ per cent held 

 mechanically. The products in the dry process, when working with 

 a 60-foot kiln, issue from the stack at a temperature of about 1500 F. 

 = 815 C. When a longer kiln is employed, and the trend of present 

 practice seems to be in the direction of 100-foot or even longer cylin- 

 ders, the stack temperatures will be correspondingly reduced. With 

 a 100-foot kiln it seems probable that they can be kept down at 

 least to 1000 F. 



In the wet process, on the other hand, the charge usually contains 

 about 60 per cent of water, though in a few plants this is kept down 

 to 30 or 40 per cent. The stack temperatures are, however, much 

 lower than in the dry process, ranging from about 800 F. with a 60-foot 

 kiln to 450 or so in a 100-foot kiln. This partly counterbalances the 

 loss of heat due to the high percentage of water. 



Using these data as a basis, Table 187 has been prepared to 

 show the amount of heat required for simply evaporating the water 

 from three different types of mixture, in kilns of two different lengths. 



TABLE 187. 

 HEAT USED IN EVAPORATION OF WATER. 



Heat utilized in decomposition of clay. An unknown, though prob- 

 ably small, amount of heat is required to dissociate the clayey portion 

 of the mix. No exact data on this point are known to the writer, but 



